1. Field of the Invention
The present invention relates to a current/charge-voltage convert circuit that uses an integrating circuit, and in particular, relates to a current/charge-voltage convert circuit having means to discharge the charge that has accumulated in the integrating capacitor.
2. Discussion of the Background Art
Current/charge-voltage convert circuits that use an integrating circuit are employed in current measuring devices, charge measuring devices, and other devices. These are circuits wherein, as shown in the integrating circuit 1 of FIG. 2, integrating capacitor 11 is connected between the inverted input terminal and the output terminal of an operational amplifier 10, integrating capacitor 11 is charged by current from the current source of the device under test 3, the amount of charge is measured by measuring the integrated voltage V, and the amount of current is measured by finding the change in the integrated voltage V.
When multiple tests are being performed with an integrating circuit that uses capacitor 11, it is necessary to discharge capacitor 11 for each measurement such that the integrating voltage V does not become saturated. The simplest discharge method is the method whereby switches are connected to both terminals of capacitor 11 and the switches are turned on at the time of discharge to short-circuit both terminals of capacitor 11. While this type of circuit has a simple structure, when FETs or other types of electronic switches are used as the switches, control signals are applied to the electrodes under control, which are based on the capacitance between the control electrodes and the electrodes under control, and a charge therefore is introduced to capacitor 11, making thorough discharge impossible. Moreover, although a charge is not introduced to the capacitor when mechanical switches are used, there is the problem here that the operating speed is insufficient.
Therefore, the method in JP (Kokai) 5-126,864 was proposed whereby diodes 200 and 201 are disposed on the inverted input terminal side of switch 203 and the other terminal of diodes 200 and 201 are fixed at a constant voltage, as shown in FIG. 2. By means of this type of circuit, the inverted input terminal side of switch 203 is always grounded via resistor 202 and the voltage of junction J1 is fixed at 0 V. When a potential of threshold voltage (approximately 0.6 V) or higher is applied in the forward direction, diodes 200 and 201 are in a connected state, but the inverted input terminal of operational amplifier 10 becomes the same potential of 0 V as the non-inverted input terminal, which is the same potential as junction J1. Therefore, when switch 203 is off, the diodes are in a stable, disconnected state and an inflow of current from the current source of the device under test 3 can be prevented.
However, even if switch 203 is on, a threshold voltage (approximately 0.6 V) is generated between junctions J1 and J2; therefore, when the voltage between the two terminals of capacitor 11 becomes the threshold voltage or less, diodes 200 and 201 become disconnected. Consequently, the voltage at both terminals of capacitor 11 cannot be discharged to the threshold voltage or less.
Therefore, the circuit shown in FIG. 3 was proposed (refer to JP (Kokai) 2002-221,540). By means of this circuit, a positive and a negative electrode (+V and −V) are disposed midway between junctions J1 and J2 and eight diodes 210 through 217 are disposed leading from the power source to each of junctions J1 and J2 in order to balance the voltage of junctions J1 and J2.
When switches 222 and 223 are turned on in order to discharge capacitor 11, the current from power source +V is split between the diodes 212 and 214 and current flows into power source −V through two paths, the path of diodes 210, 211, and 213 and the path of diodes 216, 217, and 215. Capacitor 11 is thoroughly discharged at this time because junctions J1 and J2 are brought to the same potential as a result of the current having traveled from power source +V through the same number of diodes.
On the other hand, when tests are being performed (when capacitor 11 is non-discharging), switches 222 and 223 are turned off and current does not flow into the diodes. Junctions E and F on the side of current source 3 of the device under test are grounded via resistors 218 and 219. Therefore, the inflow of current from the current source of the device under test 3 can be prevented, as shown by the circuit in FIG. 2.
Thus, while the circuit in FIG. 3 has an advantage in that the inflow of current from the current source of the device under test 3 can be prevented and capacitor 11 can be thoroughly discharged, its circuit structure is very complex because eight diodes are used. Moreover, it is necessary to use diodes with coinciding electrical properties in order to keep junctions J1 and J2 at the same potential.
The method whereby the circuit is simplified by eliminating diodes 212 through 215 has been considered because of the complexity of the above-mentioned circuit structure. However, when diodes 212 through 215 are eliminated, the output voltage V of operational amplifier 10 moves through diodes 216 and 217 and resistors 218 and 219 to become a flowing current. Therefore, a large reverse bias is applied to diodes 210 and 211 and a large leakage current flows to the inverted terminal side of operational amplifier 10. There is a problem in that when a charge is applied to capacitor 11 by this leakage current, the measurement accuracy deteriorates.